It is preferable for semiconductor circuits to have low power consumption in order to prolong the life of batteries for portable equipments and/or to reduce the size of heat sink components of semiconductor devices. For the purpose of reducing power consumption in semiconductor circuits, it is preferable to drive the circuits with a low power supply voltage. A system that uses a switching regulator to reduce a power supply voltage and further uses a series regulator to stabilize the reduced power supply is an effective means to supply a low power supply voltage in an efficient and stable manner.
A series regulator receives an input voltage Vin at one end of the channel of an output transistor, with the other end of the channel coupled to the load as an output terminal. The voltage appearing at this output terminal is an output voltage Vo. A control voltage applied to the control node of the output transistor is adjusted to control the output voltage Vo. In a series regulator circuit, the input voltage Vin is set equal to the sum of the output voltage Vo, a saturation voltage Vdsat of the output transistor, and a margin voltage. If Vo=2 V and Vdsat=0.15 V, for example, Vin is set equal to 2.2 V by taking into account a margin voltage of 0.05V.
A technology that changes the number of last-stage transistors in response to an output current is known to those skilled in the art as a means to stabilize a series regulator. In this configuration, the number of last-stage output transistors is increased in the case of larger current consumption, and is decreased in the case of smaller current consumption. With such an arrangement, the regulator can operate in a stable manner regardless of the amount of an output current.
When further reduction in the power consumption of semiconductor circuits is required in the future, it will be preferable to reduce the power consumption of a series regulator. In order to reduce the power consumption of a series regulator, a voltage difference (Vin−Vo) between the input voltage Vin and the output voltage Vo may need to be reduced. In this case, the output transistor does not operate in the saturation region, but operates in the linear region.    [Patent Document 1] Japanese Patent Application Publication No. 2003-235250    [Patent Document 2] Japanese Patent Application Publication No. 2006-190021    [Patent Document 3] Japanese Patent Application Publication No. 2006-190020    [Patent Document 4] Japanese Patent Application Publication No. 2001-282371    [Patent Document 5] Japanese Patent Application Publication No. 2005-107948